AB PLC 5 Source Or Sink

4-20mA signals are definitely "active" or "passive". Us analog guys have trouble thinking of them as sourcing or sinking.

Active 4-20mA signals are powered from an internal power supply in the device. Connect a load to an active output and the current flows. Active devices are 4 wire devices; 2 wires for power, 2 wires for the 4-20mA signal. Some people consider an active 4-20mA output to be a sourcing signal, because the 4-20mA signal requires no external power supply and the current flows as soon as a continuity in the loop is established. This is very typically an output on a chemical analyzer.

Passive 4-20mA signals are powered by an external power supply in series in the loop circuit, using the signal wires for power wires. Without an external power supply in the loop wiring, no signal is available from the transmitter.

This is done to avoid running more than 2 wires over long distances and to avoid ground loop problems that can occur when the ground for the power supply in a 3 or 4 wire device is at a different ground potential than the receiver device.

A loop powered 2 wire transmitter might be called a sinking device since it controls the current signal but that's sort of a stretch for a definition.

Could a receiver device with a 4-20mA input, like a panel meter, recorder or PLC AI be called a sinking device, because it consumes voltage in the loop? Pretty weak . . .

Here's graphic of the 3 types of wiring, 2 wire, 3 wire and 4 wire:

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differntiate properly between active & passive sink & sourcing signal 2 wire & 4 wire tx

Thanks for the informative response.

This gives me better understanding how active and passive works.

For simplicity per your explanation 4 wire is active while 2 wire is passive, which from my original question as the 3 wire instrument has a selection (dip switch), I should select sourcing (active).

There's no pat answer. Wiring is specific to the AI card and the field device.

What brand and model field instrument? Link to its manual with power requirements and a wiring diagram: www. ? ?

Which model A-B analog input card? 17__-____ ? ? ? ? ?

What other kinds of field devices are connected to the A-B input card? 2 wire transmitters on the same power supply?

Can the power supply for existing field devices support yet another field device?